Gateway GT5654 will not power on

I have a Gateway GT5654 desktop and one day it just stopped powering on. So far i have taken the power supply out and tested it within another computer, it was fine. I have also replaced the CMOS Battery. Even went to the extent of removing thermal paste from processor and putting new paste in. The Gateway just won't do anything, except for *sometimes* the MoBo fan running for a couple seconds. Maybe a bad power switch? Bad MoBo? I have searched several sites, and there was no exact answer to this. Anyone have any thoughts, experiences, fixes for this issue. Thanks in advance!

didn't have to go to that extreme because i switched around the cables and now the power switch turns the ps on, ps fan on, cpu fan on but that is all. is there something that i am missing? could it be a cpu?

i would say that it is the onboard video card. The PSU is working fine on two seperate machines. It is possbiel that i can put in a new video card within a slot and the pc will work, or will it not work because the onboard card is damaged?